1. Field of the Invention
The present invention relates to a disk reproducing method used with an optical pickup and providing high-speed reproduction, and a disk reproducing apparatus based on this method. More particularly, the present invention relates to a disk reproducing method and apparatus suitable for preventing excessive actuator heat generation of a pickup, such heat generation being caused by track following/focusing during disk eccentricity or planar vibration.
2. Description of Related Art
The CD (Compact Disk) player has long been in practical use. Recently, a so-called CD-ROM apparatus using a CD storage medium has been applied as a personal computer peripheral device, and such CD-ROM arrangement has been rapidly gaining popularity. Over time, a speed of the CD-ROM apparatus has increased because of an ongoing requirement for higher data transfer rates. Today's mainstream trend is toward a 4.times. speed (i.e., four times the speed of an original standard speed) CD-ROM as standard equipment, and in the near future, 8.times. speed and even higher speed machines are expected.
FIG. 5 is a basic block diagram illustrating a servo system for tracking control and focusing control of a disadvantaged pickup arrangement. An actuator 40 receives an electrical signal V.sub.2 to output a mechanical displacement x. The magnitude and orientation of x are detected by a detector 41 to provide an electrical signal V.sub.1. V.sub.1 is compared with a reference electrical signal 42 (V.sub.ref) in a comparator 43 to provide a differential voltage V.sub.e, which is appropriately amplified by an amplifier 44 to be fed back to an actuator 40.
FIG. 6 is a schematic diagram illustrating the principle of an optical pickup for reproducing a disk. Reference numeral 1 indicates an optical disk which rotates around an axis 50. A laser diode 51 operates as a light source. The light beam from the light source travels a path along a collimator lens system 52, a beam splitter 53, a mirror 54, and a tube 55 including an objective lens system (not shown), to thus focus on a recording surface of the disk 1. The reflected light beam from the disk 1 travels a return path along the tube 55, mirror 54 and mirror 53, to form a light spot on a light detector 41. The tube 55 is supported by a spring 56 and attached with a coil 57. Because the magnetic field of a magnet 58 reacts with any magnetic field generated by the coil 57, flowing a current through the coil 57 moves (i.e., positions) the tube 55. More particularly, as a result of the spring-56/coil-57/magnet-58 arrangement and other positioning actuator arrangements (not shown, but known in the art), a moving (i.e., positioning) force can be generated in both a tracking direction (left and right) and a focusing direction (up and down), e.g., to position the tube 55 (and objective lens; not shown) to follow a disk eccentricity 100 and/or a planar vibration 102, respectively. The CD-ROM actuator for controlling these two directions is provided with two coils. FIG. 6 shows only one coil for simplicity and clarity of illustration and description. Namely, FIG. 6 shows a model of a linear actuator.
FIG. 7 is a diagram describing an output waveform of the detector 41 in relation to a surface of the disc 1. As background, information can be recorded on the surface of the disk 1 in many ways. In FIG. 7, information is recorded by providing recesses called pits on the recording surface. The output of the detector 41 increases or decreases according to a shift of the light spot to the left or the right from the center or zero point of a track 59. This increase or decrease occurs every time the light spot traverses an adjacent track, so that a detector output such as 60 is generated.
A tracking servo is provided to hold the light spot at the center of a selected track 59. If tracking gets out of control, for example, due to an eccentricity 100 (FIG. 6) of the disk 1 or rotating system, due to a track meandering or due to a shock applied to the apparatus, a positive or negative error signal occurs in the detector output 60. For correction, the tracking servo attempts to get the light spot back to the center of a selected track based on the error signal. Accuracy of this centering action is proportional to the open-loop gain of the servo system.
Although not shown, the actuator for the CD-ROM is a two dimensional arrangement, for example, the tube 55 is also driven in focusing direction. From the detector 41, a positive or negative error signal is outputted depending on an up or down deviation from a focus of the objective lens relative to the disk. According to this error signal, a focus servo attempts to focus the light spot back to an optimal focal point.
In performing linear positioning correction and/or focussing correction using actuators as discussed above, a high work load is imposed upon the actuators. Meanwhile, in increasing the CD-ROM speed to 8.times. or higher, heat generation of the pickup actuator caused by the system cyclically compensating for disk eccentricity 100 or planar vibration 102 presents a significant problem. More specifically, causing actuators to follow the extreme cyclical disk eccentricity 100 or planar vibration 102 of a disk rotating at an 8.times. or higher speed requires a high degree of cyclical positioning with the actuators; high physical movement results in high heat generation and/or power consumption.
More particularly, the above-mentioned problem will be described in greater specificity as follows. FIGS. 8A and 8B shows general examples of the gain and phase transition characteristics, respectively, of a tracking and focusing actuator for use in a CD-ROM apparatus. When a force is acted upon a mass (e.g., on the mass of the tube 55) supported by the spring, the gain and the phase present characteristics are indicated by 61 and 62 of FIGS. 8A and 8B, respectively. Namely, the transmission characteristic of the actuator is of a quadratic system. In FIG. 8A, f.sub.o is a lower-range resonance frequency determined by an actuator's variable mass, spring constant and frictional force. In a general actuator, f.sub.o, is about 30 Hz.
Phase lag 62 (FIG. 8B) changes drastically over f.sub.o from zero to nearly .pi. radian. In FIG. 8A, f.sub.h is a higher-range resonance frequency caused mainly by a vibration of the tube 55, such vibration further increasing the phase lag 62. The frequency f.sub.A is an exemplary frequency within a range wherein a gain is linearly decreasing at -12 dB/octave.
On the other hand, FIG. 9 shows a relationship between disk reproduction high-speed ratio N and disk motor, namely disk rotational speed. In this example, a recording/reproducing method of a CD-ROM is a constant linear velocity (CLV) method in which a linear velocity between the pickup and disk is constant, and a rotational speed varies according to disk reproducing (i.e., head radial) position. More specifically, a CLV method has a 1.2 m/sec linear velocity standard, and signal recording on the disk is restricted to an area from 25 mm to 58 mm in a radial direction of the disk. Accordingly, at a 1.times. reproduction speed, for example, the rotational frequency (speed) at a position 25 mm from the inner disk periphery is about 8 Hz, and, in contrast, is about 3 Hz at a position on the outer 58 mm periphery of the disk as shown by curve 63 in FIG. 9. At a 4.times. reproduction speed, the rotational frequency is about 32 Hz at the inner periphery and about 12 Hz at the outer periphery, i.e., as shown by curve 64 in FIG. 9. Operation at such 1.times. and 4.times. speeds allow the device to operate within the initial flat gain area of FIG. 8A.
However, when an 8.times. reproduction speed is attempted, the disk rotational frequency increases to about 64 Hz (f.sub.A point) at the inner periphery as indicated by curve 65 in FIG. 9. Namely, the lower-range resonance frequency f.sub.o of the arrangement shown in FIG. 8A is exceeded, thus causing device operation disadvantageously within the secondary attenuation area. Such operation results in an increase in power dissipation and/or heat generation of the actuator. Namely, actuator sensitivity quadratically drops in device operation over f.sub.o because of the actuator characteristic, so that, if the eccentricity and planar vibration variables of the disk are fixed, the actuator drive voltage increases by the second power of (f.sub.A /f.sub.0) as compared with a 4.times. reproduction speed.
FIG. 10 is a graph illustrating the above-mentioned increase. More particularly, a power consumed by the actuator coil for tracking and focusing up to a 4.times. reproduction speed is constant, as indicated by line 66, i.e., is constant for speeds where the disk rotational frequency does not meet or exceed f.sub.0. In stark contrast, during an 8.times. reproduction speed, the power dissipation increases conspicuously as indicated by curve 67. This increase in the power dissipation cannot be practically permitted in terms of two concerns, i.e., first, in terms of an allowable power since the actuator coil itself should be power conservative for economical use and/or long life, and second, in terms of a temperature rise subjected to the wrapped coil and the objective lens (for which a plastic lens is used) which is very closely located to the actuator coil. More particularly, an excessive and unacceptable temperature rise is caused by the actuator coil which may damage/destroy the actuator coil and/or objective lens.
One solution to the above-mentioned problem is to provide a stiffer spring which holds the actuator so as to increase f.sub.o over the maximum disk rotational speed of 64 Hz, by way of example. This, however, lowers a lower-area sensitivity (gain) of the actuator, providing no advantage.